1. Technical Field of the Invention
The invention relates generally to communication systems; and, more particularly, it relates to coding (either one or both of encoding and decoding) of signals within such communication systems.
2. Description of Related Art
Data communication systems have been under continual development for many years. One such type of communication system that has been of significant interest lately is a communication system that employs turbo codes. Another type of communication system that has also received interest is a communication system that employs LDPC (Low Density Parity Check) code. A primary directive in these areas of development has been to try continually to lower the error floor within a communication system. The ideal goal has been to try to reach Shannon's limit in a communication channel. Shannon's limit may be viewed as being the data rate to be used in a communication channel, having a particular SNR (Signal to Noise Ratio), that achieves error free transmission through the communication channel. In other words, the Shannon limit is the theoretical bound for channel capacity for a given modulation and code rate.
LDPC code has been shown to provide for excellent decoding performance that can approach the Shannon limit in some cases. For example, some LDPC decoders have been shown to come within 0.3 dB (decibels) from the theoretical Shannon limit. While this example was achieved using an irregular LDPC code of a length of one million, it nevertheless demonstrates the very promising application of LDPC codes within communication systems.
Typical encoding of LDPC coded modulation signals is performed by generating a signal that includes symbols each having a common code rate and being mapped to a singular modulation (e.g., a singular constellation shape having a singular mapping of the constellation points included therein). That is to say, all of the symbols of such an LDPC coded modulation signal have the same code rate and the same modulation (the same constellation shape whose constellation points have the singular mapping). Oftentimes, such prior art encoding designs are implemented as to maximize the hardware and processing efficiencies of the particular design employed to generate the LDPC coded modulation signal having the single code rate and single modulation for all of the symbols generated therein.
Certain prior art coding approaches have sought to extend modulation coding to multiple dimensions. For example, one prior art approach is to separate a binary symbol into a plurality of parts and then map each of those parts to a corresponding constellation (according to its corresponding mapping). This approach may be viewed as being a joint mapping for multi-dimensional phase modulation based on squared Euclidean distance. This prior art approach was presented with respect to TCM (Trellis Coded Modulation) in the following reference:
[1] S. S. Pietrobon, R. H. Deng A. Lafanechere, G. Ungerboeck and D. J. Costello, Jr., “Trellis-Coded Multidimensional phase modulation,” IEEE Trans. Inform. Theory, Vol. 36, pp. 63-89, January 1990. (hereinafter referred to as [1] Pietrobon et al.)
However, this approach does not work well for coded modulation with block codes such as LDPC codes. As such, there is a need in the art to provide a workable solution for the combination of multi-dimensional phase modulation that also allows for the combination of block codes such as LDPC codes. More specifically, there is a need in the art to provide a solution that allows the benefits of LDPC codes to be combined with multi-dimensional phase modulation.